Upgrading

Upgrading from a previous version to JEM 4 requires a procedure to follow. It's not as simple as 1,2,3, please read the following instructions carefully:

1) Preparations

Make a backup!
Update to Joomla! 3.10.12
Update to JEM 2.3.6 (premigration version), if not already done.
Make a new backup!
If you have template overrides for JEM: save them. You will have to adapt them after the update. Then disable them in the template folder (renaming them e.g. from "mod_jem" to "mod_jem_backup" is sufficient)
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Disable all JEM extensions, component, all JEM plugins, all JEM modules.
DON'T uninstall!
(During upgrade process, it's suggested to switch to Joomla3 default template "Protostar")

2) Update to Joomla 4 / 5

To prepare the update from Joomla 3 to Joomla 4 and 5, go to Components > Joomla! Updates, choose Options and in Update Channel choose:
Joomla Next
Save

You'll get now the informations about which other components are not ready to migrate. Perhaps you have to disable/uninstall other components/modules/plugins not prepared for Joomla 4/5)

Update to php 8+
Then upgrade from Joomla 3 to Joomla 4 and Joomla 5.

3) Installation of JEM 4

Install JEM 4 (via upload or web installer)
Before switching to the frontend, change the positions of the JEM-modules and the main menue module, to the positions of your new Joomla4 template (e.g. the Joomla4 default template "cassiopeia")

If you get a red window, go to configuration > site > SEO and disable URL rewrite (htaccess)

Enable all JEM 4 (component, modules, plugins)
Adapt the JEM settings (there are changes: e.g. datetime format)
Adapt the JEM template overrides of necessary and reactivate them

Check everything, if it works!
Finished